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Progress in Organic Coatings 77 (2014) 1169–1183

Contents lists available at ScienceDirect

Progress in Organic Coatings

j o ur na l ho me pa ge: www.elsev ier .com/ locate /porgcoat

nti-corrosion hybrid coatings based on epoxy–silicaano-composites: Toward relationship between the morphologynd EIS data

hsan Bakhshandeha,∗, Ali Jannesari a, Zahra Ranjbarb, Sarah Sobhania,ohammad Reza Saeba

Department of Resin and Additives, Institute for Color Science and Technology, Tehran, IranDepartment of Surface Coating and Corrosion, Institute for Color Science and Technology, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 19 May 2013eceived in revised form 17 March 2014ccepted 3 April 2014

eywords:poxy–silicaol–gelybrid coatingorphology

lectrochemical impedance spectroscopyEIS)

a b s t r a c t

This work reports on design and manufacture of organic–inorganic hybrid coatings based on diglycidylether of bisphenol A (DGEBA) epoxy resin pursuing hydrolyzation of tetraethoxysilane (TEOS) through asol–gel process. The resulting hybrid materials were cured to be used as potential anticorrosive coatings.The assigned materials were modified molecules made of DGEBA and 3-aminopropyl triethoxylsilane(APTES), in which the molar ratio of epoxide group of DGEBA to NH of APTES varied in the order of2:1, 4:1, 8:1 and 16:1. In the next stage, the APTES-modified DGEBA precursors were added to differentamounts of pre-hydrolyzed TEOS, i.e. 7.5, 12.5 and 17.5 wt%, as inorganic part of the resulting hybrid. Themixtures were subsequently cured at room temperature by a cycloaliphatic amine based curing agentto yield transparent epoxy–silica hybrid coatings. Microstructure assessment of the hybrid materials,before and after curing, was performed using FTIR and 29Si NMR spectroscopies. The morphology of theepoxy–silica hybrid coatings has also been studied by scanning electron microscopy (SEM). The anti-corrosive measurements on the resultant coatings were conducted based on electrochemical impedancespectroscopy (EIS). The mechanical properties evaluation such as micro-hardness measurements andpull-off adhesion tests of the cured samples were also carried out. The thermal properties of the cured

hybrid coatings were evaluated using thermogravimetric analysis (TGA). The results showed that theconcentration of APTES and pre-hydrolyzed TEOS play an important role in determining the morphologyas well as the mechanical and thermal properties of coatings. The EIS results corresponding to these effectsreaffirmed that the corrosion resistance of the hybrid coatings improved with increasing the inorganicphase content.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Corrosion is a phenomenon through which a metal deterioratess a consequence of its interaction with the surrounding envi-onment that helps for an electrochemical reaction via oxidation.o protect or slow down the corrosion process, metals are oftenoated to yield a protective barrier against corrosive environment

1–4]. In recent years, explore and development of hybrid coatingsrabbed the attention of many researchers and engineers providinghem the opportunity of designing appropriate primers or in situ

∗ Corresponding author. Tel.: +98 21 22956209; fax: +98 21 22947537.E-mail addresses: bakhshandeh-e@icrc.ac.ir, bakhshandehehsan@yahoo.com

E. Bakhshandeh).

ttp://dx.doi.org/10.1016/j.porgcoat.2014.04.005300-9440/© 2014 Elsevier B.V. All rights reserved.

pretreatment coating systems for the protection of metal sub-strates with outstanding characteristics compared to their organiccounterparts [5–11]. These materials, denoted as organic–inorganichybrids, typically contain co-continuous domains having dimen-sions ranging in 5–100 nm [12–17]. In this regard, a wide varietyof applications were demonstrated for multifarious fields, e.g. rub-bers, plastics, sealants, fibers, optical materials, medicals, and highthermal resistance materials [18–26].

The hybrid coatings are generally prepared by low-temperaturesol–gel processes through in situ hydrolysis followed by conden-sation of organometallic precursors like silicates, titanates and

aluminates, in an organic matrix [27]. Careful selection of a hybridcoating allows combining the desirable properties of organic partof system, i.e. toughness and elasticity with those of inorganicphase that is characteristic of good hardness, chemical resistance,

1 Organ

abocaTaiaccoaierhmbbctmrpaiaerItsbimaitacidaeomarott(mloAciccmmema

170 E. Bakhshandeh et al. / Progress in

nd adhesion to the metal substrate via formation of covalentonds [28–30]. Frings et al. [31] synthesized hybrid coatings basedn polyesters and tetraethoxysilane (TEOS) to meet protectiveoatings exhibiting performance enhancement in prefinished steelnd aluminum constructions. They have shown that increasing theEOS content in the coatings causes an increase in the hardnessnd glass transition temperature (Tg). Cho and Lee [32] stud-ed the influence of in situ generated silica obtained from TEOSnd found that the mechanical properties of polyurethane basedoatings were largely governed by both the presence and con-entration of TEOS within the hybrid. Sailer and Soucek reportedn dependency of thermo-mechanical properties on the type andmount of inorganic part in a series of hybrid coatings preparedn the presence of an oxidizing alkyd [33]. It was also elucidatedlsewhere that hybrid coatings based on polyurea/polysiloxaneeveal higher adhesion to the metal substrate after addition of pre-ydrolyzed TEOS into the coatings [34]. Among family of hybridaterials, epoxy–silica composites were examined by a num-

er of researchers [35–38]. In nature, epoxy resins are famousecause of their excellent properties, e.g. superior chemical andorrosion resistance, good adhesion, and cureability at ambientemperature. Nevertheless, epoxy resin suffers from inferior ther-

al and weathering stabilities, poor mechanical properties, limitedecoating times, unsuitable cutting and welding properties. Thisuts limits on the selection of this resin for high-performancepplications [36–38]. To resolve this situation, it is suggested toncorporate TEOS inorganic precursor into the epoxy binder tochieve epoxy–silica hybrid materials [39,40]. There are sufficientvidences for creation of a phase-separated silica-rich network rep-esenting poor compatibility with epoxy resin while using TEOS.n this context, the literature addresses different investigationso meet compatibility in the epoxy–silica hybrids systems viaol–gel process [13–16,39,40]. It is a typical that as the compati-ility between the organic and inorganic parts in a hybrid system

ncreases, the solubility of the inorganic precursor in the organicatrix increases. Researchers have mostly been attempting to find

nd examine different strategies for compatibilization assessmentn the epoxy–silica system. Hence, they suggested functionaliza-ion of epoxy by functional trialkoxysilane [35,36] or simultaneousddition of this component with a coupling agent to guaranteehemical or strong physical interaction between epoxy and sil-ca domains [39,40]. Though epoxy–silica hybrid coatings wereeemed to exhibit anti-corrosive potential, there is no direct reportimed at this target. The idea of preparation of silane-modifiedpoxy resins from pre-hydrolyzed TEOS was consequently devel-ped and examined in this context. Since TEOS in the form ofonomer has a very low vapor pressure of about 1.5 mmHg at

mbient temperature, therefore, it can easily evaporate duringoom-temperature curing of the hybrid system. To prevent evap-ration of TEOS, we used its pre-hydrolyzed form and changedhe weight ratio of 3-aminopropyl triethoxysilane (APTES) to func-ionalized epoxy resin based on diglycidyl ether of bisphenol ADGEBA). The silane-modified epoxy resins were characterized by

eans of Fourier transform infrared (FTIR) and Epoxy Equiva-ent Weight (EEW) measurements. Three different concentrationsf pre-hydrolyzed TEOS as inorganic phase were added to thePTES-modified DGEBA reactive agent. The mixtures were thenured at room temperature by a cycloaliphatic amine based cur-ng agent to obtain transparent epoxy–silica hybrid coatings. Thehemical structure and morphology of the cured coatings wereharacterized using FTIR and scanning electron microscopy (SEM)ethods. Besides, mechanical properties of the coatings such as

icro-hardness and adhesion strength were evaluated. Eventually,

lectrochemical impedance spectroscopy (EIS) and thermogravi-etric analysis (TGA) were used to assess the corrosion resistance

nd thermal behavior of the organic–inorganic hybrid coatings. In

ic Coatings 77 (2014) 1169–1183

this way, EIS values are correlated this morphological features ofthe resulting coatings.

2. Materials and methods

2.1. Materials

A liquid DGEBA epoxy resin having EEW of 191 g/equiv. wasobtained from KZPC (Iran) and used as the organic phase precursor.The inorganic part of hybrid system, i.e. TEOS, was purchased fromFluka (USA). APTES coupling agent as well as tetrahydrofuran (THF)and ethanol solvents were all obtained from Merck (Germany).Hydrochloric acid (37 wt% in water) pH regulator was purchasedfrom Aldrich (USA). A cycloaliphatic amine curing agent, known asEpikure F205, was obtained from Hexion (USA). Hydrogen equiv-alent weight of this hardener was 102–104 g/mol, viscosity in therange of 500–700 mPa s, according to supplier. All other reagentsused in modification process were used as received.

2.2. Instrumentation

Infrared absorption spectra were collected on a Perkin-ElmerSpectrum One spectrometer. For carrying out this test, hybrid curedsamples were grinded and amalgamated with dry IR-grade KBr ina mortar to form pellets. Bruker DRX 500 Avance spectrometerwas used to record 29Si NMR. The morphology of the samples wasobserved using a Philips XL30 scanning electron microscope (SEM).The turbidity of the mixtures containing modified DGEBA and pre-hydrolyzed TEOS was determined by 2100 AN Turbidimeter HACH.The haziness of the cured hybrid films was measured by CE-7000Aspectrophotometer (ASTM 1003). Rheological behavior of silane-functionalized epoxy resins was investigated utilizing MCR 300Anton Paar. The micro-hardness was conducted on a HX-100 Vick-ers Micro-hardness in accordance with ASTM-E384. The test outputwas reported taking average of hardness of 3–5 points for each coat-ing. In practice, it was a difficult task identifying diagonal indentionof various hard brittle coatings due to the fractures around theindention point. Pull-off adhesion test was carried out utilizingDefelsko positest AT (ASTM-D4541), where the dollys, 20 mm indiameter, were glued firmly to the surface of the coated panelsusing a cyano-acrylate adhesive. After complete curing of the adhe-sive, the fixture was loaded and the joint was strained at a constantrate of 5 mm/min using the pull-off testing equipment, until a plugof the coating material was detached from the substrate surface.For each test sample, five replicas were employed and the averagevalue reported. Electrochemical impedance spectroscopy (EIS) wascarried out in a three-electrode system made by Ivium Compactstat(Netherlands) equipped with Ivium Equivalent Circuit Evaluatorsoftware, which enabled for determining the equivalent circuit anddata analyzing. During EIS test, the frequency range was variesbetween 100 kHz and 10 mHz, while the perturbation voltage wasset to 10 mV. The saturated calomel electrode and graphite rod wereused as reference electrode and auxiliary electrode, respectively.The fabricated coatings were applied to substrates made of steel (ST37). Before casting, steel plates were cleaned by acetone and driedto remove any kind of grease. The substrates were subsequentlyrinsed with distilled water to avoid contamination. In this manner,approximately 1 cm2 area of epoxy–silica hybrids with a thicknessof 140 ±5 �m casted on metal and exposed to 3.5% NaCl electrolyte.The rest of surface was covered with a 75/25 beeswax/colophonymixture. Thermal degradation of the coatings was investigated with

a Perkin-Elmer 6 thermogravimetric analyzer (TGA) at a heatingrate of 10 ◦C/min. The specimens of about 6–10 mg were heatedfrom room temperature to 600 ◦C under nitrogen atmosphere andtemperature-dependent weight loss was recorded.

E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183 1171

Table 1Experiments data of pull-off adhesion test for studied epoxy–silica hybrid coatings.

Samples Epoxy–silica Solvent (THF wt%) Pull-off adhesion(psi)(±0.5)

Silane functionalized DGEBA wt% TEOS pre-hydrolyzed wt%

E16T7.5 92.5 (E16) 7.5 75 3.33E16T12.5 87.5 (E16) 12.5 75 4.02E16T17.5 82.5 (E16) 17.5 75 2.71E8T7.5 92.5 (E8) 7.5 75 3.54E8T12.5 87.5 (E8) 12.5 75 4.06E8T17.5 82.5 (E8) 17.5 75 2.81E4T7.5 92.5 (E4) 7.5 75 3.01

2

vitwwtactntpei

2

fiThogb6arphwwmt

W

TE

E4T12.5 87.5 (E4) 12.5

E4T17.5 82.5 (E4) 17.5Pure epoxy 100 (unmodified epoxy) 0

.3. Preparation of pre-hydrolyzed TEOS

The preparation of pre-hydrolyzed TEOS precursor is achievedia a stepwise procedure. First, TEOS (208.33 g, 1 mol) was dissolvedn ethanol (176.62 g, 3.84 mol) and distilled water (9 g, 0.5 mol) washen added into the mixture under mechanical stirring. After theater dissolved, hydrochloric acid (0.001 molar) was added dropise and the reaction continued at 23 ◦C for 48 h. It is no denying

hat pH is a key in controlling gel structure. It is documented thatn acidic media will speed up the hydrolysis reaction more effi-iently compared to condensation reaction, for the latter involveshe attack of silicon atoms carrying protonated silanol species byeutral Si OH nucleophiles [24]. The last step was to removal ofhe solvent at 1 mmHg and 50 ◦C under vacuum to gain 144.58 gre-hydrolyzed TEOS. The yield of reaction based on TEOS and wasstimated to be 69.4% [24]. The resulting precursor was character-zed by 29Si NMR.

.4. Preparation of epoxy–silica hybrid coatings

A 25 wt% solution of silane-functionalized epoxy in THF wasrst prepared followed by addition of a mixture of pre-hydrolyzedEOS in water (3:1 molar ratio of water to TEOS). Organic–inorganicybrid coatings were formulated by addition of different amountsf the pre-hydrolyzed TEOS to the silane-functionalized DGEBA, asiven in Table 1. The next step was the addition of HCl solution toring the pH in the range of 2–3. The reactants were stirred for about

h until the mixture became clear and then the curing agent wasdded. The cycloaliphatic amine curing agent used in stoichiomet-ic ratio to cure thin films through casting the mixtures on steelanels, and the films were conditioned at 25 ◦C and 50% relativeumidity to achieve cured films containing silica domains after twoeeks exposure. It was found that dry to touch time for all samplesas approximately about 4–7 days from exposure. The stoichio-etric amounts of curing agent (WCA) were calculated evaluation

he functionalized epoxy resin, as follow:

CA = WEP × A

EEW(1)

able 2xperimental and theoretical EEW and turbidity measurements after functionalization of

Samples E16

Epoxide groups (in DGEBA) 16

Active Hydrogen atoms (in APTES) 1

Measured EEW (g/mol) 208.2

Calculated EEW (g/mol) 217.5

Turbidity (NTU)a 1.81

a Turbidity in NTU of the mixtures of APTES modified samples with 12.5 wt% of pre-hyd

75 3.8975 2.2375 5.20

where WEP is the amount of the functionalized epoxy resin, A theamine value of the curing agent (104 g/equiv.). Before incorporationof curing agent, the turbidity of the samples was determined. Theturbid-to-clear transition of the mixtures took place 4 h after addi-tion of HCl solution. The FTIR, SEM, and haziness measurementswere employed for characterization of produced hybrid materials.

3. Results and discussion

3.1. Epoxy modification assessment

The functionalization of epoxy resins with APTES was used asa route for compatibilization and control of phase separation phe-nomenon in the epoxy–silica hybrid system. The epoxy resin hasbeen to a specified extent functionalized to generate an epoxy sys-tem with the potential to cure with cycloaliphatic amine used inthis work. Thus, some samples revealed insufficient compatibilityas a consequence of improper curing under stoichiometric ratio.The modification reaction with the silane component was carriedout varying molar ratio of epoxide groups to active hydrogen atomsof amine group of APTES, as given in Table 2. By choosing differentstoichiometric ratios of epoxide to amine groups; it was possible totrack both the crosslink ability and applicability of resulting mate-rial as a coating. The chemical interactions between the epoxy resinand APTES were studied by FTIR and EEW measurements. Fig. 1shows the absorption spectra of pure APTES, DGEBA, and silane-modified DGEBA samples. The reaction was supposed to take placein accord with Scheme 1 suggesting that functionalization of epoxywith alkoxysilane takes place through the reaction between theamine groups of APTES and the epoxide groups of DGEBA. Thecharacteristic absorption bands for the NH and Si OEt (Et denot-ing Ethyl) bonds of APTES appeared at 3381, 1605 and 1099 cm−1,respectively. Also, the absorption peak at 916 cm−1 relating toepoxide groups is weakened during the functionalization reaction.The presence of the peak of OH group near the 3400 cm−1 makesevident that epoxide ring opening reaction between DGEBA and

APTES is occurred. The theoretically calculated and determinedEEW values of functionalized epoxides are listed in Table 2. As canbe seen, there is a good agreement between theoretical and exper-imental investigations implying an increase in the values of EEW

the epoxy resin.

E8 E4 E2

8 4 21 1 1

232.6 288.4 –240.3 295.1 Gel formation

1.37 1.03 –

rolyzed TEOS in functionalization of the epoxy resin.

1172 E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183

e(site

l(Scibo

When the pre-hydrolyzed TEOS was added to the APTES-modified epoxy, a turbid mixture was obtained. Turbidity valuesof the mixtures of 12.5 wt% pre-hydrolyzed TEOS in different

Fig. 1. FTIR spectra of (A) DGEBA, (B) APTES, (C) E8, (D) E4, and (E) E2.

stimated for modified specimens compared to that of neat epoxy191 g/mol). During functionalization of epoxy resin with APTES,ome side reactions might also been possible, which are displayedn Scheme 2 [27,41]. Thus, depending on resin to APTES molar ratio,he contribution of Si O Si bonds to gel formation could to somextent be probable.

As shown in parts (c) and (d) in Fig. 1, with increasing APTESevel in the modification reaction the intensity of Si O C bands1080–1100 cm−1) decreases, while the one corresponding toi O Si bands (near 1030 cm−1) is associated with an increase. Vis-

osity dependence of the silane-modified DGEBA samples is plottedn Fig. 2. The high viscosity at low-shear rate region can be causedy linear extent of the epoxy chains reacting with the amine groupf APTES and also by the formation of silica domains. As can be

Scheme 1. Chemical reaction between DGEBA and APTES.

Scheme 2. The proposed side-reactions taking place during sol–gel process.

seen, there is a viscosity upturn over increasing the concentrationof APTES within the system at the assigned region moving fromsample E16 to E8 and then E4. In fact, formation of silane bondsin the system is of lesser probability in case of E16. It is obviousthat this system behaves nearly to a Newtonian fluid at the studiedshear rate interval due to shortage of inter- and intra-molecularinteractions. On the other hand, samples containing more APTESunits undergo shear thinning as a consequence of possible destruc-tive effects at higher shear rates. It is to be noted that owing to veryhigh content of APTES in the sample E2, a gelled mass was formedthat made rheological measurement impossible.

3.2. Characterization of pre-hydrolyzed TEOS

The preparation of oligomeric precursors based on TEOS throughhydrolysis followed by co-condensation reactions have been thesubject of some investigations, as speculated in Scheme 3 [42–47].Fig. 3 illustrates 29Si NMR analysis of pre-hydrolyzed TEOS exhibit-ing different chemical shifts at −89.98, −95.68 and −96.45 ppmassigning to terminal group Si O Si (OEt)3, cyclic ring Si O Si(OEt)2 O Si and linear Si O Si (OEt)2 O Si, respectively. Theresonance splitting is considered to be the superposition of thespecies with similar structure and different degree of polymeriza-tion.

3.3. Compatibility analysis

Fig. 2. Viscosity as a function of shear rate and molar ratio of epoxide group to NHgroup in the APTES: 16/1, 8/1, 4/1.

E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183 1173

Scheme 3. Preparation of pre-hydrolyzed TEOS exhibiting different chemical structures during sol–gel reaction.

Fig. 3. 29Si NMR spectra of (A) TEOS monomer and (B) pre-hydrolyzed TEOS.

1174 E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183

les (A

Am(dop(fio

3

3

iaattTe

3

d

Fig. 4. Optical micrographs of the mixtures of APTES modified-epoxy samp

PTES-modified DGEBA samples, i.e. E16, E8, and E4, wereeasured and expressed in nephelometric turbidity units, NTU

Table 2). As can be seen, the turbidity of the mixture has risen overecreasing the amount of APTES in the samples. Optical microscopybservations, as represented in Fig. 4, reveal that the size of the dis-ersed phase (the pre-hydrolyzed TEOS) in the continuous phasethe APTES-modified DGEBA) varies by the APTES level. This reaf-rmed turbidity measurements implying enhanced compatibilityver APTES content.

.4. Characterization of hybrid coatings

.4.1. Cure assessmentThe FTIR analysis shows a very high reactivity between epox-

de and amine functional groups after two weeks exposure atforementioned condition (Fig. 5). This was evidenced by the dis-ppearance of absorption band at around 916 cm−1 correspondingo epoxide groups. The Si O Si groups can also be detected inhe hybrid films at wavelength in the range of 1100–1200 cm−1.o make a deeper sense on formation of covalent bond betweenpoxy–amine network and silica domains, one can see Scheme 4.

.4.2. Haziness and morphologyAll cured samples formed a slightly hazy transparent film, but

ifferent in haziness values. The haziness values of the hybrid films

Fig. 5. FTIR spectrum of E8T12.5 s

: E4, B: E8, C: E16) with 12.5 wt% pre-hydrolyzed TEOS (scale = 31.76 �m).

are measured and reported in Fig. 6. As can be seen, the hazinessof the films has increased as a result of a rise in the content ofpre-hydrolyzed TEOS. At a given concentration of TEOS; however,the higher APTES content leads to more compatibility betweenorganic and inorganic phases followed by formation of smaller silicadomains, thereby lesser haziness of hybrid films.

Fig. 7 compares SEM micrographs of the hybrid samples con-taining 12.5 wt% of pre-hydrolyzed TEOS in APTES modified-DGEBAand the one relating to neat epoxy. Accordingly, the silica particlessmaller than 100 nm in size are uniformly dispersed throughoutthe polymer matrix of the sample E8T12.5 (see Table 1). Thisreveals good miscibility between organic and inorganic parts inthe nanocomposites. To make a greater sense of compatibility,distribution pattern of the silica domains is determined by Pixca-vator Image Analyzer, as in Fig. 8. It is shown that silica particlesformed throughout the matrix have an average size in the range of20–40 nm. This places emphasis on development of a nanostructurebased upon proposed scheme in this investigation. Another impor-tant aspect can be found comparing the SEM images of E4T12.5and E16T12.5. Accordingly, larger silica domains in E16 before cur-ing might have been the reason for morphology coarsening in case

E16T12.5 when compared with the other cured film. In the otherwords, the final size of the silica domains depends on their initialsize achieved in the pre-hydrolytic stage, as evidenced by opticalmicroscopic study.

ample after curing reaction.

E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183 1175

Scheme 4. Chemical reaction between DGEBA–APTES and pre-hydrolyzed TEOS.

Fig. 7. SEM image of epoxy–silica hybrids. (A) E4T12.5

Fig. 6. Haziness of the cured films of samples; effects of APTES amounts (E16:16/1,E8:8/1, S4:4/1) and pre-hydrolyzed TEOS (7.5, 12.5, 17.5).

3.4.3. Mechanical propertiesTo evaluate the adhesion strength of hybrid coatings, pull-off

adhesion test was carried out on the hybrid coatings as well as neatepoxy coating applied on mild steel surface. The results of measure-ments are given in Table 1. Accordingly, the hybrid epoxy–silicacoatings wholly show weaker adhesion strengths, approximatelyfrom 22 to 58% lower in comparison with the neat epoxy coating. Itis known that the presence of APTES in epoxy matrix has beneficialaspects regarding adhesion characteristics because of acceptableSi O metal interface [27]. On the other hand, the presence of TEOS

in the hybrid coating causes formation of a silica layer near thesurface of substrate, thereby metal-coating adhesion might be suf-fered due to formation of microcracks within the cured films. As

, (B) E8T12.5, (C) E16T12.5 and (D) pure epoxy.

1176 E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183

F

cniasttlsclo

gscn

Fdacivfttpi

ig. 8. The size distribution of silica domains in epoxy–silica hybrid (E8T12.5).

an be seen in Table 2, manipulating the content of APTES doesot seriously affect the adhesion strength of hybrid coatings hav-

ng the same amount of TEOS (one can compare E4Tx with E8Txnd E16Tx, when x = 7.5, 12.5, and 17.5). Evidently, the adhesiontrength is enhanced between 30 and 40% with increasing concen-ration of TEOS from 7.5 to 12.5 wt%, but followed a descendingrend at concentration of 17.5 wt%, regardless of APTES content. Theowest value of pull-off data among hybrid coatings is devoted toample E4T17.5. This was expected in view of population of micro-racks in this sample. Thus, very high concentration of Si O metalinkages could be a cause for adhesion strength depression in casef hybrid coatings with respect to neat epoxy coatings.

To place value on this speculation, we provided SEM micro-raphs from the detached surface of E8T12.5 from the mild steelurface exhibiting the highest pull-off value in this family ofoatings (Fig. 9). This image clearly shows formation of silica layerear the substrate.

The results of micro-hardness measurements are plotted inig. 10. The micro-hardness of the hybrid coatings appears to beependent on the pre-hydrolyzed TEOS content. The maximumttainable hardness in this series is 12.3 HV corresponding to thease that 12.5 wt% of TEOS is used, i.e. E4T12.5 sample. By furtherncreasing in TEOS content from up to 17.5 wt%, the micro-hardnessalues follow a descending trend. This can be ascribed to lack of uni-ormity and improper distribution of silicate domains throughout

he film. On the other hand, increasing the APTES content leadso compatibility enhancement between the organic and inorganichases showing micro-hardness amelioration. Fig. 11 presents SEM

mages of the surface of E8T12.5. SEM micrograph clearly shows

Fig. 9. SEM and graphic images of epoxy–silica hybrid coating, th

Fig. 10. Micro-hardness of the hybrid coatings: effects of APTES amounts (E16:16/1,E8:8/1, E4:4/1) and pre-hydrolyzed TEOS wt% (7.5, 12.5, 17.5).

that silica nanoparticles are evenly distributed on the surface ofthe sample.

3.4.4. EIS studiesElectrochemical impedance spectroscopy (EIS) is well-known

for intensively prediction of corrosion resistance. Impedance indi-cates changes at the interface of coating and metal long beforevisual traditional exposure tests can indicate [45,46]. EmployingEIS results, it is possible to deeply delve anticorrosive character-istics of prepared coatings. We used Bode and Nyquist plots foridentification of coating performance and better description of thecoating impedance behavior. The impedance spectra were ana-lyzed using impedance in low frequency (lZl0.01 Hz) which is anindicator of coating condition under continuous immersion [47].Besides, an electrochemical equivalent circuit model was suggestedto correlate EIS variations with corrosive environments. Model(a) in Fig. 12 is simply illustrative of Rsol and Rpor respectivelyresistance caused by solution, and passive film, while Qcoat standsfor the constant-phase element for the passive film. This circuitwas designed to fit all the experimental results from the hybridcoatings after half an hour of immersion in the sodium chloridesolution. Also, circuit (b) was used to fit the experimental dataafter 15 and 45 days of immersion in the salt solution. In this

circuit, Rpor is the pore resistance of coating, Qcoat the constant-phase element for the passive film, RlSi the resistance caused bysilica layer possibly formed in the neighborhood of metal, QlSi theconstant-phase element for silica layer capacitance, Rpol the

e detached surface of E8T12.5 from the mild steel surface.

E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183 1177

y–sili

cdcshaos

ieso(aTfi

4ftEdt

Fa

Fig. 11. SEM and graphic images of epox

harge-transfer resistance, and Qdl the constant-phase element forouble-layer capacitance at the coating-substrate interface. In bothircuits, Rsol is the electrolyte resistance. The typical impedancepectra of the steel substrate coated with hybrid films after half anour, 15 days and 45 days of immersion in 3.5 wt% NaCl solutionsre compared in Figs. 13, 15 and 17. Fig. 13 shows the Bode plotsf hybrid coatings after half an hour of immersion in 3.5 wt% NaClolution.

Employing the EIS data relating to epoxy–silica hybrid coatingsmmersed for half an hour, the Rs (Qsol Rpor) model was applied onquivalent circuit model to make a fit on single passive film pre-ented on the metal surface (see Fig. 14). The corrosion resistancef the hybrids on steel was evaluated by the coating resistanceRpor) and impedance at low frequency region (lZl0.01 Hz) after halfn hour of immersion in a 3.5 wt% NaCl solution. Accordingly toable 3, there is a good agreement between the experimental andtted values.

In a similar manner, Figs. 15 and 17 are obtained after 15 and5 days of immersion in a 3.5 wt% NaCl solution and revealed aall in the impedance values to 107–108 � and 105–106 �, respec-

ively. In these plots, two time constants are obviously seen in theIS spectra of the Bode plots over an immersion time increase,emonstrating three corrosion regions. This may be assigned tohe formation of defects caused by penetration of ionic species

ig. 12. (a) Randles equivalent circuit of intact coating (b) proposed electrical equiv-lent circuit for corroded hybrid coated metals.

ca hybrid coating, the surface of E8T12.

and water through the films. As more water diffuses into the filmduring immersion, the film resistance declines and finally waterfinds its path to the substrate and accelerate corrosion reactions.The proposed electrical equivalent circuits for the epoxy–silicahybrid coatings corresponding to 15 and 45 days of immersion areshown in Figs. 16 and 18, respectively. As can be seen, the resis-tance of coatings decreases by more than one order of magnitudewith increasing the immersion time. However, each hybrid coat-ing exhibited similar trend by the time to failure (Figs. 15 and 17)as determined from low frequency depression in impedance withrespect to immersion time. The higher amounts of pre-hydrolyzedTEOS and APTES obviously accelerated downward trend in the rateof coating resistance for the hybrid coatings immersed in NaCl solu-tion (Fig. 19). This behavior could be attributed to higher contentsof silica content in the films. For drawing comparison betweenexperimental and fitted data, we provided Tables 4 and 5. In thisregard, we simulated a parallel arrangement of a resistor and acapacitor for the silica layer near the substrate. The resistance ofgrowing silica layer near the substrate, Rlsi, increases with increas-ing the TEOS content, whereas the capacitance Clsi, decreases. Thistrend was expected taking into account growing silica domainsin the assigned substrates. A comparison between the impedancespectra of different samples after 15 and 45 days of immersiontest revealed that the resistances and capacitances of the hybridcoatings decreased and increased respectively after 45 days ofimmersion (see Figs. 15 and 17). These alterations are indeed simi-lar to the results obtained after 15 days of immersion; thereby, therate of water uptake and the hydrolytic stability of coatings duringimmersion in aqueous solutions can be determined from the evo-lution of the capacitance of the hybrid films [48]. The water uptakevalues summarized in Table 6 are obtained from Brasher–Kingsburyequation (2):

Water uptake = log(Ct/C0)log 80

(2)

where Ct and C0 are the electrical capacitance of the coating duringand before the immersion, respectively [46]. The results indicatethat water uptake of hybrid coatings decreases as the silica con-centration increases. These results confirm that by incorporationof pre-hydrolyzed TEOS in the hybrid coatings the corrosion resis-tances enhances, meanwhile, water uptake decreases. It can beconcluded that silica domains might have been formed as a barrierlayer inside the film, which prevent the penetration of electrolyteand decrease the corrosion rate. This is confident with SEM obser-

vations in Fig. 9. The condensed SiO2 layer on the substrate was alsodeemed to be able to block the electrolyte path leading to corro-sion inhibition. In this context, increasing the pre-hydrolyzed TEOScontent has positive impacts on anticorrosive characteristics.

1178 E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183

Fig. 13. Bode plots for different samples after half an hour of immersion (A) E16TX, (B) E8TX, (C) E4TX; (♦) X = 7.5 wt%, (�) X = 12.5 wt% and (�) X = 17.5 wt%.

2.5 ep

3

tct2i

TS

Fig. 14. Fitted Bode and Nyquist impedance spectra of E8T1

.4.5. TGA analysisThermogravimetric analysis was performed to investigate

he thermal-decomposition behavior of the epoxy–silica hybrid

oatings. The thermograms (Fig. 20) show a two-step degrada-ion mechanism. It goes without saying that the weight loss below00 ◦C is attributable to evaporation of volatiles ingredients used

n the hybrid system, which might have been remained because

able 3elected circuit fitting results for elements as shown in Fig. 1(a), after half an hour of imm

Samples Qcoat (F/cm2) Rpor

E16T7.5 2.2 × 10−10 1.77 × 109

E16T12.5 2.0 × 10−10 3.81 × 109

E16T17.5 1.8 × 10−10 5.42 × 109

E8T7.5 2.3 × 10−10 1.58 × 109

E8T12.5 2.0 × 10−10 4.106 × 109

E8T17.5 1.0 × 10−10 4.55 × 109

E4T7.5 7.0 × 10−11 3.017 × 109

E4T12.5 6.2 × 10−11 4.448 × 109

E4T17.5 2.3 × 10−11 2.073 × 109

oxy–silica hybrid coatings after half an hour of immersion.

of incomplete polycondensation reaction that is residual silanols.On the other hand, the wide interval of the weight loss within200–600 ◦C might be attributed to decomposition of polymeric part

of the coatings. Table 7 provides thermal properties of pure epoxyresin and hybrid samples in terms of some important tempera-tures expecting from TGA test. It is obvious that the initial thermaldecomposition temperature (Td) of the epoxy–silica hybrids is

ersion.

lZl at 0.01 Hz Phase angles at 104 Hz n

1.81 × 109 89.01 0.953.91 × 109 89.52 0.945.45 × 109 89.76 0.961.59 × 109 89.67 0.944.2 × 109 89.14 0.934.65 × 109 89.24 0.963.03 × 109 88.56 0.944.45 × 109 88.83 0.952.73 × 109 88.97 0.95

E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183 1179

Fig. 15. Bode plots for different samples after 15 days of immersion, (A) E16TX, (B) E8TX, (C) E4TX; (♦) X = 7.5 wt%, (�) X = 12.5 wt% and (�) X = 17.5 wt%.

T12.5

hdbtd[

TS

Fig. 16. Fitted Bode and Nyquist impedance spectra of E8

igher than that of the pure epoxy. The pure epoxy sample starts toecompose at 320 ◦C, while the hybrids were possibly decomposed

◦ ◦

etween 325 C and 342 C. These observations can be attributedo the strong interaction between the polymer chains and silicaomains, which prevents thermal decomposition of the hybrids41,49]. It should be mentioned that fluctuation in Td would be

able 4elected circuit fitting results for elements as shown in Fig. 15(b), for studied samples aft

Samples Rpor Qcoat RlSi

E16T7.5 3.5 × 107 3.2 × 10−10 2.0 × 107

E16T12.5 4.2 × 107 2.5 × 10−10 2.2 × 107

E16T17.5 5.0 × 107 2.1 × 10−10 8.6 × 107

E8T7.5 2.4 × 107 3.3 × 10−10 2.2 × 107

E8T12.5 2.5 × 107 2.7 × 10−10 4.0 × 107

E8T17.5 7.3 × 107 2.1 × 10−10 7.0 × 107

E4T7.5 2.0 × 107 1.0 × 10−10 1.2 × 107

E4T12.5 2.5 × 107 2.1 × 10−10 5.0 × 107

E4T17.5 7.1 × 107 1.0 × 10−10 7.5 × 107

epoxy–silica hybrid coatings after 15 days of immersion.

attributable to the degree of compatibility between phases in thehybrid coatings, while the ash content accounts for concentra-

tion of inorganic silica domains remained after TGA test. Sinceincrease of TEOS in the system causes more incompatibility, Tdcould be suffered at high TEOS contents. Ash content of epoxy andhybrid samples are measured at 1000 ◦C to evaluate contribution of

er 15 days of immersion.

Qlsi Rp Qdl lZl at 0.01 Hz

3.0 × 10−6 8.0 × 106 3.0 × 10−7 4.91 × 107

1.0 × 10−8 1.0 × 107 1.0 × 10−6 6.52 × 107

5.0 × 10−8 5.2 × 107 2.0 × 10−7 2.21 × 108

9.5 × 10−9 8.8 × 106 1.0 × 10−8 5.41 × 107

3.0 × 10−8 2.0 × 107 1.0 × 10−8 9.74 × 107

5.0 × 10−8 1.0 × 107 1.0 × 10−8 1.64 × 108

1.1 × 10−8 2.0 × 107 2.9 × 10−8 4.91 × 107

1.0 × 10−7 4.0 × 107 1.0 × 10−8 7.53 × 107

2.0 × 10−7 8.2 × 107 1.0 × 10−7 1.73 × 108

1180 E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183

Fig. 17. Bode plots for different samples after 45 days of immersion, (A) E16TX, (B) E8TX, (C) E4TX; (♦) X = 7.5 wt%, (�) X = 12.5 wt% and (�) X = 17.5.

Fig. 18. Fitted Bode and Nyquist impedance spectra of E8T12.5 epoxy–silica hybrid coatings after 45 days of immersion.

Table 5Selected circuit fitting results for elements as shown in Fig. 15(b), for studied samples after 45 days of immersion.

Samples Rpor Qcoat RlSi Qlsi Rp Qdl lZl at 0.01 Hz

E16T7.5 1.02 × 105 2.1 × 10−9 1.0 × 104 2.0 × 10−3 1.0 × 104 2.0 × 10−3 1.19 × 105

E16T12.5 1.30 × 105 1.6 × 10−9 8.1 × 104 1.1 × 10−3 3.0 × 104 1.0 × 10−4 2.10 × 105

E16T17.5 5.31 × 105 1.0 × 10−9 8.6 × 104 1.1 × 10−4 3.2 × 104 1.0 × 10−5 7.55 × 105

E8T7.5 1.12 × 105 7.4 × 10−9 4.2 × 104 1.2 × 10−6 2.0 × 104 1.0 × 10−4 1.09 × 105

E8T12.5 6.12 × 106 3.2 × 10−9 1.0 × 106 2.0 × 10−6 2.0 × 105 1.0 × 10−5 6.92 × 106

E8T17.5 7.21 × 106 1.5 × 10−9 1.3 × 106 4.1 × 10−7 1.0 × 105 1.0 × 10−6 8.81 × 106

E4T7.5 1.12 × 106 8.5 × 10−10 5.2 × 105 2.5 × 10−7 3.0 × 105 2.0 × 10−5 1.02 × 106

E4T12.5 2.42 × 106 6.5 × 10−10 1.0 × 106 1.5 × 10−7 5.0 × 104 2.0 × 10−5 3.42 × 106

E4T17.5 6.10 × 106 2.1 × 10−10 2.1 × 106 1.0 × 10−7 1.2 × 105 2.0 × 10−5 1.05 × 107

Table 6Water uptake values as a function of TEOS wt% for hybrid coatings.

V.F.H2O Sample

E16T7.5 E16T12.5 E16T17.5 E8T7.5 E8T12.5 E8T17.5 E4T7.5 E4T12.5 E4T17.5

After 15 days 0.223 0.051 0.033 0.211 0.147 0.045 0.116 0.084 0.017After 45 days 0.515 0.475 0.391 0.793 0.633 0.618 0.570 0.537 0.509

E. Bakhshandeh et al. / Progress in Organic Coatings 77 (2014) 1169–1183 1181

Fig. 19. Impedance in low frequency (lZl0.01 Hz) changes during immersion time (day) as determined by circuit fitting over 0.5 h and 15, 45 days of immersion. (A) ExT7.5, (B)E

iaca

xT12.5, (C) ExT17.5; (♦) X = 16, (�) X = 8 and (�) X = 4.

norganic silica domains to the coatings. All hybrid specimens have

sh contents in the range of 2.33–6.83 wt% in the order that TEOSontent is increased. Regarding the compatibility between organicnd inorganic parts of the sample, it can be inferred that increasing

Fig. 20. TGA curves of the hybrid coatings. (A) E16

the content of APTS coupling agent causes a shift in T and coating

dpossessing higher compatibility are formed. For example, increas-ing Td from 332 to 342 moving from E16T7.5 toward E8T7.5 can beconsidered.

TX, (B) E8TX, (C) E4TX; X = 7.5, 12.5 and 17.5.

1182 E. Bakhshandeh et al. / Progress in Organ

Table 7The initial degradation temperature (Td) and weight loss percent of epoxy–silicahybrids.

Samples Td (◦C) Total decrease ofwt% at 1000 ◦C

E16T7.5 332 97.67E16T12.5 325 96.29E16T17.5 328 94.98E8T7.5 342 97.52E8T12.5 341 96.12E8T17.5 333 94.83E4T7.5 329 96.88

4

etctmIitrmpdptrtwleT1wadatedgactucbcc

A

c

R

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[

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[[

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E4T12.5 341 95.99E4T17.5 327.1 93.17Pure epoxy 320 100

. Conclusions

Hybrid coatings based on silane functionalized diglycidylther of bisphenol A (DGEBA) epoxy resin and pre-hydrolyzedetraethoxysilane (TEOS) were prepared and comprehensivelyharacterized. In the studied hybrid systems, we changed the con-ent of 3-aminopropyl triethoxysilane (APTES) coupling agent to

ake organic and inorganic parts of hybrid system compatible.t was found that increasing the APTES content causes a viscos-ty upturn at low shear rates. Confidently, FTIR spectra illustratedhat the shear thinning behavior of the silane-functionalized epoxyesins can be attributed to the formation of silica domains. Theorphological studies placed value on this finding indicating the

resence of a slightly hazy transparent hybrid film with nanoscaleimensions. The haziness measurements showed that the com-atibility between the organic and inorganic phases depends onhe APTES content, as sample E4 (epoxide/amine ratio of 4:1)evealed smaller silica domains among all prepared samples. Onhe other hand, the haziness of hybrid films has obviously risenith increasing the amount of pre-hydrolyzed TEOS in the formu-

ations. It was also observed that adhesion strength of samplesnhances in the range of 30–40% by weight over increase ofEOS from 7.5 to 12.5 wt%, while there was a fall assigned to7.5 wt% of TEOS. The maximum micro-hardness was 12.3 HVith devoted to sample containing 12.5 wt% TEOS, namely E4T12.5,

nd further increase in TEOS content up to 17.5 wt% caused aescending trend being observed. The EIS experimental data werenalyzed considering two equivalent electrical circuits. The fit-ed values corresponding to equivalent circuits were used tovaluate the evolution of corrosion protection properties for theifferent hybrid coatings under study. Analysis of EIS data sug-ests that the corrosion protection of epoxy–silica coatings isttributable to formation of an intermediate silica layer at theoating-substrate interface, which prevents the penetration of elec-rolyte and decreases the corrosion rate. We also served waterptake quantities to prove anticorrosive nature of fabricated hybridoatings. Thermogravimetric analysis evidenced that thermal sta-ility of epoxy–silica hybrid coatings is highly dependent on theoncentration of APTES coupling agent as well as inorganic phaseontent.

cknowledgment

The authors would like to acknowledge with gratitude Pars Pam-hal Chemical Co. who kindly supported this work financially.

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